1. I might do the Scotch Tape Test to be sure he isn't reinfected with pinworms -especially if only his tail has dandruff.
2. Buy a medicated shampoo and thoroughly wash & rinse the tail. If he's easy going, I would shampoo the tail twice. You might even use a human shampoo like Selsun Blue or whatever is available to you.
3. It could also be that his sheath needs a good cleaning. -- if you don't know when that was done, some geldings get really filthy and itchy up inside. They will rub their butt against something to try and relieve that internal itch, thus leading people to think it's their butt that itches when it isn't
Regardless of the cause, washing the tail with something medicated will help him feel a little better
My CB had been treated for pinworms before I got her and was left with a flaky, itchy tail. Once I retreated for pinworms and found no improvement I cleared it up with 1% hydrocortisone cream. It stops the itch and gave the skin a chance to heal.